The present invention relates to tanks, and in particular to railway tank cars of the manifolded type which may be interconnected to accommodate loading or unloading of an entire train of interconnected cars without movement thereof, from a single point therealong, thereby accommodating consecutive loading, transporting and unloading of fluid ladings and facilitating the formation of unit trains.
Manifolded railway tank cars have been designed with two top-located lading conduits having internal downward extensions of different lengths within the tank. The longer of the two lading conduits typically terminates closely adjacent to the bottom of the tank and permits unloading of liquid from the tank to an almost empty condition. The shorter of the two lading conduits has the inner end thereof spaced closely adjacent to the top of the tank and limits the liquid fill level when the tank is loaded through the other lading conduit. Such a manifolded tank car is disclosed, for example, in U.S. Pat. No. 3,897,807.
Typically, the tank car is designed to be filled to a maximum level which leaves a certain amount of free space or "outage" at the top of the tank above the liquid lading, typically in the range of about 2% of the total volume of the tank. This outage accommodates vapors formed above the liquid lading. The maximum fill level is determined by the level of the inner end of the shorter lading conduit, since when the surface of the liquid reaches this level it closes the shorter lading conduit and prevents the further escape of vapors therethrough. Thus, the vapors are confined to the very small volume of the "outage" and are compressed, this pressure forcing the liquid up through the shorter lading conduit and to the next tank car for filling thereof.
If a tank car is to be used to transport a specific liquid exclusively, the tank is sized so that when filled to a 2% outage the car weight is at the load limit weight. If a greater density liquid were to be transported, a lesser volume of liquid would bring the car to is load limit and, therefore, the maximum fill height would be less resulting in an outage greater than 2%. Accordingly, when a train of tank cars is to be filled automatically and consecutively from one end of the train with a particular liquid lading, it is necessary that the shorter lading conduits of each of the tanks terminate at the proper maximum fill level for that lading. Accordingly, prior tank cars have had the lading conduits thereof designed for a particular lading, so that different tank car constructions had to be used for ladings of different densities. This greatly impaired the utility of a tank car and tremendously increased the numbers of cars which had to be maintained to accommodate different types of ladings.